I've just tried it and couldn't find a problem. What browser are you using and which method? I tried both double clicking the round icon from outside the forum section and the dropdown menu working fine for me.

Rapscallion

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Perhaps the "markreadhash" is the required "token?" (I really need to brush up on my PHP, again.)

EDIT: I've also just attempted to use the "Mark Forums Read" option from the "Quick Links" using fusion_x, and still produces the shorter, unusable URL as above. Safe to assume it's something odd with the stylesheet?

EDIT II: Out of curiosity I tried this at CS; the default and "cobalt_x" styles had the "markreadhash" data in the URL, but the "Mobile" style did not, thus marking the forums read didn't work for it.